1. Field of the Invention
The present invention relates to cable snubbing. In particular the present invention is an improved method and apparatus for restraining or checking the motion of a cable used in the underwater positioning of a seismic source.
2. Disclosure of the Prior Art
In the practice of underwater seismic prospecting it is necessary to introduce beneath the water's surface a source of energy for inducing propagating acoustic waves. Generally it is desired that the source of energy, or seismic source, be positioned at a selected depth beneath the surface and maintained there during its operation. It is known in the art that one method for positioning a seismic source at a selected depth is to suspend the seismic source by one or more chains or cables beneath a float. The float is designed to remain buoyant at the water's surface while supporting the combined weights of the seismic source and the chains or cables connecting the float and the seismic source. Generally, one group of cables, the lifting cables, is used to lift and lower the float and seismic source from the deck of a ship into the water. Another group of cables, the support cables, is used to suspend the source beneath the float. When the source is in the desired underwater position, the weight of the source is borne by the support cables and the lifting cables are in a substantially no load condition.
To facilitate performing seismographic operations at several locations and to avoid removing the seismic source from underwater, it is common practice in underwater seismic prospecting to tow the float and submerged seismic source from one propsecting location to another. Towing is achieved usually by separate towing cables from the towing ship being affixed to the float and the seismic source. During a towing maneuver, the support chains remain taut but the submerged portions of the lifting cables experience a drag force as they move through the water causing the submerged portion of each lifting cable to increase. Even when the seismic source is not being towed, the wave motion causes the float to bob about such that the drag force on the submerged portion of the cable tends to pull the unsubmerged portion into the water. This reduces the amount of slack in the above surface portion of the lifting cables. Since the towing ship and the float heave and move relative to each other because of wave action, it is desirable to have slack in the above water surface portion of each lifting cable sufficient to allow for such motion and to prevent any tension from occurring in the above surface portion of the cable as the ship and float heave and move. Slack in the above water portion of the cable will prevent chafing or abrading of the float by the cable and will prevent breaking the lifting cable should the ship or float experience sudden movement relative to one another. To assure that sufficient slack remains in the above surface portion of a lifting cable, a means for checking or restraining the cable from further movement into the water is used. One such means is sometimes referred to as a cable snubber and is usually attached to the float.
The present invention is an improved method and apparatus for snubbing or checking the movement of a cable and includes using a sleeve member with flexible members extending inwardly from the sleeve wall to restrain a cable moving through the sleeve. A hollow casing with radially inwardly extending brushes is shown in U.S. Pat. No. 2,998,801 issued Sept. 5, 1961 to Arthur N. Edelberg, assignor to Kamco Products, Inc., Cleveland, Ohio. Edelberg's invention is a device for painting flagpoles or vertical suspension cables of suspension type bridges. It incorporates a casing with bristle brushes to seal the top and bottom ends of the casing from leakage of paint spray and to smooth the paint sprayed on the flagpole by paint spray guns as the device is lowered down the pole. Edelberg states that the brushes may be removed and replaced with wire brushes for removing loose paint, scale and rust from the flagpole or suspension cable about which it is raised and lowered. The Edelberg device appears to be unsuited for use in underwater prospecting applications. Further it is not a device for restraining the motion of the pole or cable with which it is used; rather it is clear from its intended use that the device moves rather easily and smoothly up and down the pole or cable to be painted.
U.S. Pat. No. 3,224,020 issued Dec. 21, 1965 to J. G. Domingos for a Golf Grip Cleansing Device discloses a cylindrical shell with cleansing brushes positioned in the upper interior portion of the shell and a quantity of water in the lower portion. The grip of a golf club may be inserted into the shell. Moving the grip up and down is said to remove perspiration, soil and other foreign material from the surface of the grip. The device would not be suited for or suggestive of a means for restraining the movement of a cable nor would it have any application to underwater mineral prospecting.
U.S. Pat. No. 3,335,469 issued Aug. 15, 1967 to Shand et al for Personal Safety Equipment discloses a device, referred to as a mobile safety anchor, which is intended to grip a safety line upon sudden acceleration and thereby prevent the fall of a workman from height. The device includes a runner through which a safety line can be threaded, balls which serve as line-gripping members and a wedge means for wedging the balls against the safety line. The device does not continuously grip or restrain the cable but does so only upon rapid acceleration of the runner.
One type of prior art cable snubber which has been used in underwater prospecting is formed from a solid cylinder of rubber or polyamide polymer, such as nylon. A hole is bored axially through the solid cylinder forming a passage with a surrounding wall. The cross sectional area of the passage usually is circular with a diameter slightly larger than, but approximately the diameter of, the cable to be restrained. A cable to be restrained is then threaded through the snubber. Since the cable diameter and bored hole diameter are approximately the same, the cable fits snugly. Any relative motion between the cable and the snubber is resisted by the frictional force generated between the outer surface of the cable and the surface of the passage through the snubber. A problem with this type of snubber is that the resistive frictional force abrades away the rubber or polymer material of the snubber wall, thereby increasing the diameter of the passage and reducing the restraining ability of the snubber.